U.S. patent number 6,951,048 [Application Number 10/700,852] was granted by the patent office on 2005-10-04 for method for producing a stacked piezoelectric element.
This patent grant is currently assigned to Canon Kabushiki Kaisha, Taiheiyo Cement Corporation. Invention is credited to Toru Ezaki, Nobuyuki Kojima, Yutaka Maruyama, Takahiro Yamakawa.
United States Patent |
6,951,048 |
Maruyama , et al. |
October 4, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Method for producing a stacked piezoelectric element
Abstract
A method for producing a stacked piezoelectric element by
alternately stacking a plurality of layers of an electrode material
and piezoelectric layers having an electro-mechanical energy
converting function and provided with penetrating electrodes, which
are obtained by forming through holes in each piezoelectric layer
and filling such through holes with the electrode material, to be
connected at a contact portion with a layer of the electrode
material and sintering the thus stacked layers, includes a step of
forming, on a first layer of the electrode material, a second layer
of electrode material by printing at a peripheral area of the
contact portion between the first layer of the electrode material
and the penetrating electrodes.
Inventors: |
Maruyama; Yutaka (Tokyo,
JP), Kojima; Nobuyuki (Kanagawa-ken, JP),
Ezaki; Toru (Tokyo, JP), Yamakawa; Takahiro
(Saitama-ken, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
Taiheiyo Cement Corporation (Tokyo, JP)
|
Family
ID: |
27288600 |
Appl.
No.: |
10/700,852 |
Filed: |
November 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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597727 |
Jun 19, 2000 |
6668437 |
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251494 |
Feb 17, 1999 |
6291932 |
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Foreign Application Priority Data
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Feb 17, 1998 [JP] |
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10-034981 |
Mar 13, 1998 [JP] |
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10-063210 |
Mar 13, 1998 [JP] |
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10-063211 |
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Current U.S.
Class: |
29/25.35; 29/830;
29/846; 29/852; 310/366 |
Current CPC
Class: |
H01L
41/0474 (20130101); H01L 41/293 (20130101); H01L
41/0833 (20130101); Y10T 29/49155 (20150115); Y10T
29/42 (20150115); Y10T 29/49165 (20150115); Y10T
29/49126 (20150115) |
Current International
Class: |
H01L
41/083 (20060101); H01L 41/00 (20060101); H01L
41/047 (20060101); H01L 41/24 (20060101); H04R
017/00 (); H01L 041/04 () |
Field of
Search: |
;29/25.35,830,852,846
;310/364,332,323.01,366,328,363,311,323.06 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Jimenez; Marc
Assistant Examiner: Nguyen; Tai Van
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Parent Case Text
This is a divisional application of application Ser. No.
09/597,727, filed on Jun. 19, 2000 U.S. Pat. No. 6,668,437, which
is a divisional of application Ser. No. 09/251,494, filed on Feb.
17, 1999, now U.S. Pat. No. 6,291,932, issued on Sep. 18, 2001.
Claims
What is claimed is:
1. A method for producing a stacked piezoelectric element by
alternately stacking a plurality of layers of an electrode material
and piezoelectric layers having an electro-mechanical energy
converting function and provided with penetrating electrodes, which
are obtained by forming through holes in each piezoelectric layer
and filling such through holes with the electrode material, to be
connected at a contact portion with a layer of the electrode
material and sintering the thus stacked layers, comprising a step
of: forming, on a first layer of the electrode material, a second
layer of electrode material at a peripheral area of the contact
portion between the first layer of the electrode material and the
penetrating electrodes.
2. A method for producing a stacked piezoelectric element by
alternately stacking a plurality of layers of an electrode material
and piezoelectric layers having an electro-mechanical energy
converting function and provided with penetrating electrodes, which
are obtained by forming through holes in each piezoelectric layer
and filling such through holes with the electrode material, to be
connected at a contact portion with a layer of the electrode
material and sintering the thus stacked layers, comprising a step
of: forming, on a first layer of the electrode material, a second
layer of electrode material, at the contact portion on a connecting
surface of the penetrating electrodes and at a peripheral portion
thereof.
3. A method for producing a stacked piezoelectric element by
alternately stacking a plurality of layers of an electrode material
and piezoelectric layers having an electro-mechanical energy
converting function and provided with penetrating electrodes, which
are obtained by forming through holes in each piezoelectric layer
and filling such through holes with the electrode material, to be
connected at a contact portion with a layer of the electrode
material and sintering the thus stacked layers, comprising a step
of: forming a second layer of electrode material which is thicker
than a first layer of the electrode material on the piezoelectric
layer at a peripheral portion of a contact area between two
penetrating electrodes.
4. A method for producing a stacked piezoelectric element by
alternately stacking a plurality of layers of an electrode material
and piezoelectric layers having an electro-mechanical energy
converting function and provided with penetrating electrodes, which
are obtained by forming through holes in each piezoelectric layer
and filling such through holes with the electrode material, to be
connected at a contact portion with a layer of the electrode
material and sintering the thus stacked layers, comprising a step
of: forming a second layer of electrode material which is thicker
than a first layer of the electrode material at a peripheral area
of the contact portion between the first layer of the electrode
material and the penetrating electrodes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stacked piezoelectric element
and a producing method therefor.
2. Related Background Art
Conventionally there has been proposed a stacked piezoelectric
element in which an electro-mechanical energy converting material
such as piezoelectric ceramics having electro-mechanical energy
converting function and an electrode material are alternately
stacked. Such stacked piezoelectric element, in comparison for
example with plate like single piezoelectric ceramics of a same
thickness, can provide a larger distortion for deforming or a
larger generated force with a lower applied voltage, and is
therefore investigated and employed for use in the driving portion
of a vibration element constituting a vibration driving device such
as a piezoelectric actuator or a vibration wave motor.
The stacked piezoelectric element is produced principally in the
following two methods.
The first producing method consists of forming electrode layers on
both surfaces of a sintered single piezoelectric ceramic plate,
stacking a plurality of such ceramic plate and adhering, for
example with adhesive material, the ceramic plates.
The second producing method is an integral sintering method
consisting of superposing and thermally pressing layers of
unsintered sheet-shaped molded member (green sheet) containing
piezoelectric ceramics and an organic binder and unsintered layers
of electrode paste and sintering thus superposed layers.
For connecting the electrode layers in thus produced stacked
piezoelectric element with the outside, there is proposed a method
of forming electric connection between the layers by a through hole
(via hole) electrode, which is a penetrating electrode obtained by
forming a through hole in each piezoelectric ceramic layer and
filling such through hole with an electrode material.
Particularly in the second producing method for the stacked
piezoelectric element, the through hole (via hole) electrode is
obtained by forming a hole in the green sheet of the piezoelectric
ceramics and filling the holes with conductive paste before
stacking, and then sintering the green sheets after stacking.
A stacked piezoelectric element, in which such through hole (via
hole) electrode is exposed on the surface layer of the stacked
piezoelectric element and is used as the electric conductive means
to the external circuit such as a printed wiring board, is proposed
for example in the Japanese Patent Application Laid-open No.
8-213664.
When such proposed stacked piezoelectric element is employed in rod
or Langevin type of a vibration wave actuator or motor, the
reference shows that the smoothness of the upper and lower surfaces
of the element affects the mechanical quality coefficient (Qm) of
the entire device.
It is therefore proposed to apply surface processing (lapping,
polishing, grinding etc.) to the upper and lower surfaces of the
stacked piezoelectric element after sintering, thereby obtaining
the element of high flatness.
The through hole (via hole) electrode of the stacked piezoelectric
element may be provided at a position arbitrarily selected for
forming electrical conduction between the layers.
For example in FIG. 6A, in a piezoelectric ceramic 2-1 of a first
layer, constituting the surface layer of the stacked piezoelectric
element 1, a through hole electrode 4-1 is formed in a position
suitable for external connection.
In a piezoelectric ceramic 2-2 of a second layer, a through hole
electrode 4-2 is provided in a position suitable for connecting the
piezoelectric ceramic 2-1 of the first layer to a piezoelectric
element 2-3 of a third layer so as to be made an electrical
connection by an electrode film 3-2.
In the third and lower layers, a through hole electrode 4-3 and
further through hole electrodes are provided in positions having
relatively little contribution to the mechanical force generation,
in order to achieve effective force generation of the stacked
piezoelectric element 1.
However, the surface processing on the upper and lower surfaces of
the stacked piezoelectric element of the above-described
configuration has revealed the necessity for further improvement as
will be explained in the following.
In the piezoelectric ceramics 2 constituting the stacked
piezoelectric element 1 and the conductive member constituting the
through hole electrodes 4 for the different layers, the green sheet
and the conductive paste containing the conductive material show
mutually different contraction rates at the sintering operation,
because of the characteristics of each material and the different
mixing ratios of the organic binder present prior to the
sintering.
For this reason, there is generated a residual stress in a hatched
area a shown in FIG. 6A. A defect from the falling off is easily
generated in such area .alpha., and it is confirmed that, if the
smoothing surface processing is applied to the upper and lower
surfaces of such stacked piezoelectric element 1, a part of the
piezoelectric ceramic drops off in an area .beta. shown in FIG.
6B.
Such dropping off of the piezoelectric ceramic exposes the through
hole electrode 4-2, leading to a drawback such as shortcircuiting
with the wiring on the printing wiring board.
On the other hand, the electrode material employed in the
above-described process is generally composed of a precious metal
(platinum, palladium, silver etc.) having high melting temperature
or a mixture thereof because it has to be sintered together with
the piezoelectric ceramic material. The electrode material is most
commonly composed of a mixture of silver and palladium with a
weight ratio of 5:5 to 8:2 though it is dependent also on the
sintering temperature of the piezoelectric ceramic.
Such precious metals are expensive and constitute the largest
proportion in the material cost of the stacked piezoelectric
element. For this reason, the electrode layers are formed as thin
as possible within the manufacturable range of the stacked
piezoelectric element and within the acceptable performance range
thereof, by the improvement in the electrode paste consisting of
the electrode material, organic binder, solvent and other additives
and the layer forming method such as screen printing.
The piezoelectric element thus formed is then subjected to a
polarization process for enabling elongation and contraction for
the actual use. The uppermost surface electrode layer is used as a
contact electrode in such polarization process but is removed after
the polarization process, thereby enabling connection of the
individual through hole (through hole filled with conductive
material being often called a through hole electrode or a via hole)
exposed on the surface with the driving circuit.
Prior to the actual use, the stacked piezoelectric element is
subjected to the polarization process of applying a voltage to the
piezoelectric ceramic layers. This process is executed by applying
a voltage of 3 to 1 kV/mm for a period of 30 to 60 minutes under a
high temperature (80.degree. C. to 200.degree. C.).
In this process, the current generated after the voltage
application becomes very large in case of the stacked piezoelectric
element because of the significantly larger electrostatic
capacitance in comparison with the conventional single-plate
piezoelectric element. Consequently a large current is generated in
the thin electrode layers of the stacked piezoelectric element,
particularly in the connecting portion between the surface
electrode layer in direct contact with the DC power source for the
polarization process and the conductive electrode connecting the
surface electrode to the internal electrode layers, whereby sparks
are often generated to induce fused breakage of the connecting
portion, thus inhibiting the polarization process or eventually
resulting in the destruction of the element by the shock of the
sparks.
The stacked piezoelectric element utilizes, in the conductive
electrode for connecting the internal electrode layers, the through
hole commonly employed in the printed circuit board, and has an
internal electrode layer (to be explained later) having a circuit
wiring function in addition to the surface electrode layer in
direct contact with the DC power source. Such wiring electrode
layer also causes sparks or fused breakage at the connecting
portion with the conductive electrode, leading to the destruction
of the device.
FIGS. 13A and 13B illustrate a stacked piezoelectric element
described in the Japanese Patent Application Laid-open No.
8-213664. The stacked piezoelectric element 11 in FIG. 13A is
composed, as shown in FIG. 13B, of n piezoelectric layers
(piezoelectric ceramic layers) 14 (14-1 to 14-n). In the stacked
piezoelectric element 11, the second and subsequent piezoelectric
layers (14-2 to 14-n) are respectively provided with electrode
layers 13 (13-2 to 13-n) for example of 4-divided configuration,
and the electrode layers requiring mutual conduction are connected
by through holes 12 penetrating through the piezoelectric
layer.
More specifically, each of the first to (n-1)th piezoelectric
layers 14-1 to 14-(n-1) is provided with eight through holes, and
the first through hole 12-1-1 of the first piezoelectric layer 14-1
is connected to the first electrode 13-2-1 of the second electrode
layer while the third through hole 12-1-3 of the first
piezoelectric layer 14-1 is connected to the third through hole
12-2-3 formed in the second piezoelectric layer 14-2 and to the
second electrode of the third electrode layer on the third
piezoelectric layer 14-3.
Subsequently the first through hole formed in each piezoelectric
layer is similarly connected to every even numbered layers, to the
through hole 12-(n-2) of the third piezoelectric layer from the
bottom and to the first electrode layer 13-(n-1)-1 on the second
piezoelectric layer 14-(n-1) from the bottom. The third through
hole in the third and subsequent piezoelectric layers is connected
similarly to every odd numbered layers, and to the second electrode
formed on the lowermost piezoelectric layer 14-n.
SUMMARY OF THE INVENTION
One aspect of the invention is to provide a stacked piezoelectric
element in which a plurality of piezoelectric layers and electrode
layers are alternately stacked and thus stacked electrode layers
are connected by a penetrating electrode formed in a conductive
hole provided across the piezoelectric layer, wherein the
penetrating electrode formed in the second piezoelectric layer is
so positioned as to overlap with that formed in the first
piezoelectric layer in the direction across the piezoelectric
layer, thereby improving the reliability.
One aspect of the invention is to provide a stacked piezoelectric
element in which the surface electrode layer utilized in the
polarization process is made thicker than the internal electrode
layers, thereby reducing defects caused in the polarization process
and improving the production yield.
One aspect of the invention is to provide a stacked piezoelectric
element in which, in the junction portion between the through hole
and the electrode material layer, there is provided a reinforcing
electrode material layer for increasing the thickness of the
electrode material layer larger in the peripheral portion of the
through hole than in other portions, thereby reducing the defects
caused in the polarization process and increasing the production
yield.
Other objects of the present invention, and the features thereof,
will become fully apparent from the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a stacked piezoelectric element
constituting a first embodiment of the present invention;
FIGS. 2A and 2B are cross-sectional views of the stacked
piezoelectric element of the first embodiment of the present
invention;
FIG. 3 is a table showing the thickness of the stacked
piezoelectric element of the first embodiment of the present
invention;
FIG. 4 is a table showing the thickness of the stacked
piezoelectric element of a second embodiment of the present
invention;
FIGS. 5A and 5B are cross-sectional views of the stacked
piezoelectric element of the second embodiment of the present
invention;
FIGS. 6A and 6B are cross-sectional views of a conventional stacked
piezoelectric element;
FIG. 7 is a perspective view of a stacked piezoelectric element
constituting a third embodiment of the present invention;
FIG. 8 is a plan view of the layers constituting the stacked
piezoelectric element shown in FIG. 7;
FIG. 9 is a partial lateral cross-sectional view of a stacked
piezoelectric element constituting a fourth embodiment of the
present invention;
FIG. 10 is a perspective view of a fifth embodiment of the present
invention in the polarization process;
FIG. 11 is a plan view of the layers constituting the stacked
piezoelectric element shown in FIG. 10;
FIGS. 12A and 12B are partial lateral cross-sectional views of the
stacked piezoelectric element, respectively in the fifth embodiment
in a conventional configuration; and
FIGS. 13A and 13B are views showing a conventional stacked
piezoelectric element, respectively in a perspective view and in an
exploded perspective view showing different layers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment.
FIGS. 1, 2A, 2B and 3 illustrate a first embodiment of the present
invention.
FIG. 1 is an external view of the stacked piezoelectric element of
the present embodiment, and FIGS. 2A and 2B are cross-sectional
views thereof.
In the present embodiment, the green sheet is prepared, in
consideration of the shrinkage in the sintering, in such a manner
that the thickness after sintering becomes for example 0.087 mm. In
the actual manufacturing process, it is confirmed that the
thickness of the green sheet after sintering shows a variation of
.+-.0.002 mm though the range of variation is dependent on the
manufacturing process to a certain extent.
The above-mentioned green sheet is subjected to the formation of
through hole electrodes 4 (numbered as 4-1 in the surface layer, as
4-2 in the second layer thereunder and 4-3 in the third layer), an
electrode film 3 (that on the first layer being used as the contact
terminal for the polarization process of the stacked piezoelectric
element but being removed thereafter, that on the second layer
being numbered as 3-2, that on the third layer as 3-3 and that on
the fourth layer as 3-4) etc., then press formed, cut into a
desired shape and sintered.
In the present embodiment, there are employed 7 layers.
After processing, the stacked piezoelectric element has a dimension
for example with an outer diameter of 10 mm, an inner diameter of 4
mm and a thickness of 0.5 mm.
The outer diameter after sintering is selected as about 11 mm in
consideration of the dimension after processing, and is finished as
10 mm by grinding.
FIGS. 2A and 2B show the cross section in the surface portion in
the radial direction (center at the right hand side in figure) of
the through hole electrode (via hole) of the stacked piezoelectric
element 1, respectively before and after the lapping of the both
surfaces.
In the present embodiment, as shown in FIGS. 2A and 2B, the through
hole electrode 4-2 of the second layer is so positioned as to be
projected, in the axial direction, in the same position as the
through hole electrode 4-1 of the first layer (namely so as to
mutually overlap in the axial direction).
The through hole electrodes 4-3 etc. of the third and subsequent
layers are not particularly limited in the present invention.
The thickness after sintering varies by the bending in the
sintering, variation in the shrinkage rate, variation in the
thickness of the green sheet etc.
In the present embodiment, lapping on both surfaces is executed
after the polarization process as shown in FIG. 2B, in order to
correct the variation in the thickness and to secure the smoothness
of the upper and lower surfaces.
The thicknesses before and after the lapping of the both surfaces
of the stacked piezoelectric element 1 are shown in FIG. 3.
In order to secure the smoothness and to obtain a constant
thickness, there is required a processing allowance shown in FIG.
3. In case the first layer is formed with a thickness of 0.089 mm
as in a sample C of a piezoelectric element shown in FIG. 3, the
thickness from the upper surface of the element to the interface
between the first and second layers becomes 0.0275 mm which is
significantly smaller than in samples A and B.
In the present embodiment, the through hole electrode 4-2 of the
second layer is so positioned as to be projected in the same
position as the through hole electrode 4-1 of the first layer, so
that the residual stress present in the layers on the through hole
electrode 4-3 of the third layer can be dispersed and there can be
provided a stacked piezoelectric element of excellent reliability
without dropping of the piezoelectric ceramic resulting from the
difference in the shrinkage rate between the green sheet and the
conductive paste.
Second Embodiment.
FIGS. 4, 5A and 5B illustrate a second embodiment.
The outer and inner diameters in the present embodiment are same as
those in the first embodiment and will not be explained further.
The variation in the thickness of the green sheet is also same as
in the first embodiment. In the present embodiment, there are
employed 27 stacked layers.
The electrode film 3 and the through hole electrodes 4 formed in
each layer are schematically shown in FIGS. 5A and 5B.
FIGS. 5A and 5B respectively show states before and after the
lapping of the two surfaces. As shown in FIG. 5A, the through hole
electrode 4-2 of the second layer is so as to be projected, in the
axial direction, in the same position (namely so as to overlap) as
the through hole electrode 4-1 of the first layer.
The through hole electrode 4-3 of the third layer is so formed as
to be in the same position, in the projection, as the through hole
electrode 4-2 of the second layer, and is therefore in the same
position, in the projection, as the through hole electrode 4-1 of
the first layer.
The thickness before and after the lapping of the both surfaces of
the stacked piezoelectric element of the present embodiment are
shown in FIG. 4.
As the stacked piezoelectric element of the present embodiment has
27 stacked layers with a large variation of thickness after
sintering, there may be required a lapping allowance (on one
surface) as large as 0.1 mm as in a sample C shown in FIG. 4. As
this lapping allowance is larger than the thickness 0.089 mm of the
first layer, the surface after processing is constituted by the
second piezoelectric ceramic layer 2-2 at the sintering as shown in
FIG. 5B.
In the present embodiment, the through hole electrode 4-3 formed in
the third layer is provided, in the projection, in the same
position as the through hole electrode 4-2 of the second layer, so
that there can be prevented the dropping of the piezoelectric
ceramic of the second layer, resulting from the residual stress in
the portion present under the through hole electrode 4-4 of the
fourth layer.
In the first and second embodiments, similar effect can be obtained
by lapping, polishing or grinding on one or both surfaces.
In the first and second embodiments, the through hole electrode
(via hole), formed as a penetrating electrode in the conductive
hole of the piezoelectric ceramics such as the surfacial
piezoelectric member, overlaps with the through hole electrode (via
hole) of the next layer in the direction across the layer.
Therefore, even when the surface processing is applied to the
surface of the first layer or executed down to the surface of the
second layer, there can be prevented the partial dropping defect
resulting, in a position opposed to the through hole electrode,
from the difference in the shrinkage rate between the piezoelectric
ceramics and the electrode and there can be obtained a stacked
piezoelectric element with improved reliability.
Third Embodiment.
FIGS. 7 and 8 illustrate a third embodiment of the present
invention.
FIG. 7 is an external view of the stacked piezoelectric element 1
at the polarization process, and FIG. 8 illustrates the uppermost
conductive electrodes or through holes 13 (13-1 to 13-12)
connecting the surface electrode layer 15 (including electrode
sections 15-1 to 15-3) of the first layer to the electrode layers
16-2 of the second layer and subsequent electrode layers (16-2 to),
and the conductive electrodes 14 (14-2-1 to 9 . . . ) of the second
and subsequent layers.
The stacked piezoelectric element of the present embodiment for
example has an outer diameter of 10 mm, an inner diameter of 2.8 mm
and a thickness of about 2 mm and includes a first electrode layer
(diameter 9.5 mm) and second to twenty-second electrode layers
(diameter 9 mm) which are formed by screen printing electrode paste
on green sheets (not shown) of a thickness of 90 .mu.m, consisting
of powdered piezoelectric ceramics and an organic binder.
The through holes 13, 14 are obtained by forming a hole of about
0.1 mm in the green sheet and filling the hole with the electrode
paste, in order to obtain electrical conduction between the
electrode layers. The electrode paste contains silver and palladium
in a weight ratio of 6:4. The green sheets and the electrode layers
are precisely superposed, then mutually adhered by hot pressing,
and then sintered with a maximum temperature of 1120.degree. C.,
with shrinkage of about 20% at the sintering.
The through holes 13-2 to 13-9 are connected from the first to
twenty-third electrode layers, while the through hole 13-1 is
connected to the third layer (electrode layer S in the third layer
being used for a signal outputting sensor), and the through holes
13-10 to 13-12 are used for recognizing the position of the present
stacked piezoelectric element.
The electrode layers 15-1 to 15-3 of the first layer are surface
layers directly receiving the voltage by the contact pins 12 from
the polarizing DC power source as shown in FIG. 7. The polarization
process was executed by employing a DC power source 18 in the
configuration shown in FIG. 7 and applying a voltage of +180 V at
the (+) side and -180 V at the (-) side with respect to the ground
GND for 1 hour in silicone oil of 150.degree. C.
In the present embodiment, the first electrode layer 15 is made
thicker than other electrode layers 16.
The first to twenty-second electrode layers, containing expensive
precious metals, have conventionally been formed as thin as
possible, but the obtained thickness is more than 1 .mu.m at
minimum and is generally 2 to 3 .mu.m in average, based on the
currently available electrode paste and screen printing method.
If the polarization process is executed in this state under the
above-described conditions, a current is abruptly generated
immediately after the voltage application as in charging of a
capacitor (the present stacked piezoelectric element has an
electrostatic capacitance of 65 .mu.F at (+) or (-) side), thereby
easily causing sparks in the junctions between the through hole
electrodes 13-2 to 13-9 and the electrode layers 15-1 to 15-3,
eventually resulting in defective polarization caused by fused
breakage or destruction of the element by the shock of the sparks,
as explained in the foregoing.
Such defects have been almost removed when the thickness of the
surface electrode layers 15-1 to 15-3 is increased to 4 to 6 .mu.m
in average by changing the printing screen.
Such effect becomes securer by further increasing the thickness of
the surface electrode layer 15. It is presumed that the increase in
the thickness of the surface electrode layer achieves securer
conduction with the through hole surface and the reduced electrical
resistance reduces the possibility of fused breakage by the sparks.
However an excessive thickness is undesirable because of the
increased cost.
The present embodiment employs the conductive electrode formed by
the through hole, but a similar result based on the increased
thickness of the electrode layer can also be obtained with a
lateral face electrode, which is a conductive electrode formed on
the lateral face of the device.
Fourth Embodiment.
In the present embodiment, the second electrode layer 16-2 shown in
FIG. 8 as well as the first electrode layer 16-1 are made thicker
than other electrode layers.
The second electrode layer 16-2 is an internal electrode layer with
conductive circuit function, in which through holes 13-4, 14-2-4;
13-5, 14-2-5; 13-7, 14-2-7; 13-8, 14-2-8; and 13-9, 14-2-9 in five
positions are connected to the through holes of the third layer
through the second electrode layer.
FIG. 9 is a schematic cross-sectional view, in the radial
direction, of the through holes 13-4 and 14-2-4. The through hole
13-4 connected to the electrode layer 15-1 used for polarization is
connected to the through hole 14-2-4 through the electrode layer
16-2 of the second layer.
The increased thickness of the electrode layers reduces the
electrical resistance in the joints between the through hole 13-4
and the electrode layer 15-1, between the through hole 13-4 and the
electrode layer 16-2 and between the through hole 14-2-4 and the
electrode layer 16-2, thereby achieving securer conduction and
avoiding spark generation.
On the other hand, the through holes 13-2, 13-3 and 13-6 connected
straight to the twenty-second layer. The through holes 13-1, 13-10
to 13-12 are same as explained before.
The thickness of the second electrode layer is also conventionally
selected as 2 to 3 .mu.m in average, but the current abruptly
generated after the start of the polarization process tends to
generate sparks, resulting frequently in defective polarization,
crack formation on the surface of the first layer or destruction of
the entire element. Such defects are almost eliminated by
increasing the thickness of the second electrode layer to 4 to 6
.mu.m in average.
However, with a further increased thickness of the electrode layer,
the first electrode layer becomes easily peelable from the formed
piezoelectric ceramic layer because the second layer is an internal
electrode layer. It is presumed that the electrode layer is
basically free from chemical reaction with the piezoelectric
ceramic layer so that the peeling tends to occur when the electrode
layer becomes thicker. For this reason, the thickness of the second
circuit forming electrode layer is optimally within a range of 4 to
6 .mu.m.
The second electrode layer need not necessarily be made thicker in
the entire electrode layer but can be made thickness only in the
areas of the through holes 13-4, 14-2-4; 13-5, 14-2-5; 13-7,
14-2-7; 13-8, 14-2-8; 13-9, 14-2-9. However, in consideration of
the screen printing technology employed in the production, it is
easier to increase the thickness of the second electrode film in
the entire area thereof.
The above-described stacked piezoelectric element can be applied as
the vibration generating source constituting a vibration member in
a vibration driving device such as a vibration motor, in which a
traveling wave is generated by the synthesis of bending vibration
in two directions, but the application is not limited to such
object. It can also be utilized as the vibration generating source
for other purposes with an appropriate change in the configuration
of the electrode layers. It is for example sufficiently applicable
to a stacked piezoelectric transducer.
As explained in the foregoing, the third and fourth embodiments
reduce the defects caused in the polarization process, thereby
improving the production yield of the stacked piezoelectric
element, and enable the polarization process within a shorter time,
without particular change in the polarizing power source, voltage
condition thereof, or method or means for voltage elevation.
Fifth Embodiment.
FIGS. 10 and 11 illustrate a fifth embodiment of the present
invention.
FIG. 10 is an external view of the stacked piezoelectric element 1
at the polarization process. The stacked piezoelectric element 1 of
the present embodiment for example has an outer diameter of 10 mm,
an inner diameter of 2.8 mm and a thickness of about 2 mm.
FIG. 11 illustrates the surface electrode layer 25 (including
electrodes 25-1 to 25-3) of the first piezoelectric of the stacked
piezoelectric element 21 and the surface electrode layers 26 (26-2
to 26-23) of the second and subsequent piezoelectric layers, and
the positions of the through holes (indicated by black circles)
connecting these layers, including the through holes 23 (23-1 to
23-12) of the first layer and those 24 (24-2-1 to 9 . . . ) of the
second and subsequent layers.
In the present embodiment, the first electrode layer has an outer
diameter of 9.5 mm and an inner diameter of 2.8 mm, while the
second to twenty-third electrode layers 26-2 to 26-23 have an outer
diameter of 9 mm and an inner diameter of 3.4 mm.
The stacked piezoelectric element of the present embodiment is
formed by utilizing green sheets (not shown) of a thickness of
about 85 .mu.m consisting of powdered piezoelectric ceramics and an
organic binder, and the first electrode layer 25 and the second and
subsequent electrode layers 26 are formed by screen printing
electrode paste on the green sheets.
The through holes 23, 24 of a diameter of 0.1 mm are obtained by
forming holes in the green sheet and filling the holes with the
electrode paste by screen printing, in order to obtain electrical
conduction between the electrode layers. The green sheets, the
electrode layers and through holes are precisely superposed, then
mutually adhered by hot pressing, and then sintered with a maximum
temperature of 1120.degree. C.
Among the through holes, the hole 12-1 formed in the first
piezoelectric layer is connected to a sensor electrode S (for
detecting the vibration state in case of application in a vibration
motor) formed in the third electrode layer, while the linearly
arranged through holes 23-10 to 23-12 are used for position
confirmation, and the through hole 23-9 is aligned with these
position confirming through holes and connected to the second
electrode layer. Other through holes 23-2-23-9 are connected from
the first electrode layer 25 to the twenty-second electrode layer
26-22 or the twenty-third electrode layer 26-23.
As shown in FIG. 10, the first electrode layers 25-1 to 25-3
directly receive the voltages of the polarizing DC power source 28
by the contact pins 22-1 to 22-3. The polarization process was
executed by employing a DC power source 28 and applying a voltage
of +180 V at the (+) side and -180 V at the (-) side with respect
to the ground GND for 1 hour in silicone oil of 150.degree. C.
Conventionally the first electrode layer 25, the second electrode
layer 26-2 and the subsequent twenty-third electrode layer 26-23
are made as thin as possible, with an average thickness of 2 to 3
.mu.m based on the currently available electrode paste and screen
printing method.
FIGS. 12A and 12B show the cross section of the through holes along
the stacking direction, respectively in the stacked piezoelectric
element of the present embodiment shown in FIG. 11 and in the
conventional stacked piezoelectric element, illustrating the first
electrode layer 25, the through hole 23-4 therein, the through hole
24-2-4 of the second layer, the through hole 24-3-4 of the third
layer, the through hole 24-4-4 of the fourth layer, the through
hole 24-5-4 of the fifth layer, the through hole 24-22-4 of the
twenty-second layer and the lowermost twenty-third electrode layer
26-23. The through hole 23-4 of the first layer and the through
hole 24-2-4 of the second layer have different distances from the
center of the element but the through holes of the second and
subsequent layers have a same distance from the center of the
element.
In the junctions between the electrode layers and the through holes
of the stacked piezoelectric element of the present embodiment,
there are provided thickness increasing electrode layers 27-3-4',
27-2-4, 27-2-4', 27-5-4, . . . , 27-23-4 in the vicinity of the
through holes as shown in FIG. 12A, thereby increasing the
thickness of the electrode layer around the through hole in
comparison with the conventional configuration shown in FIG.
12B.
Also in the junction between the through holes of the second and
third layers, a reinforcing electrode layer is provided on the
surface of the third piezoelectric layer, around the through holes
therein, so as not contact the electrodes formed on such surface,
and similar reinforcing electrode layers 27-3-4, 27-4-4, 27-6-4
(not shown), . . . , 27-22-4 are formed.
As a result, the junctions between the electrodes of the electrode
layers and the through holes and those between the through holes
are mechanically and electrically reinforced by such reinforcing
electrode layers.
In the conventional configuration shown in FIG. 12B, the junctions
between the electrodes of the electrode layers and the through
holes and those between the through holes may result in defective
conduction because of the lack of such reinforcing electrode
layers, but such defective conduction can be prevented in the
present embodiment.
The above-described reinforcing electrode layer, particularly in
case of formation on the electrode layer, has a thickness of 2 to 3
.mu.m at minimum based on the currently available technologies as
in the case of the electrode layers 25, 26, but is given a
thickness of 4 to 6 .mu.m in the present embodiment for achieving
securer conduction.
The additional reinforcing electrode layer is formed by printing
electrode paste after the formation of the electrode layer for
voltage application or of the through hole.
The additional reinforcing electrode layer is desirably so shaped
that the thickness gradually increases from the peripheral portion
of the through hole to the through hole in consideration of the
mechanical and electrical reinforcing, and can be easily formed by
printing electrode paste with the ordinary screen printing method
and stacking the green sheets by hot pressing.
The additional reinforcing electrode layers as shown in FIG. 12A
realize securer conduction between the electrode layers and the
through holes and between the through holes.
The level of conduction can be evaluated by measuring the
electrostatic capacitance after the polarization process. The
stacked piezoelectric element of the present embodiment has an
electrostatic capacitance of 63 nF in average, with a variation of
about .+-.2 nF.
On the other hand, the conventional stacked piezoelectric element
has an electrostatic capacitance of 46 nF in average, with a
variation as large as 0 to 64 nF, evidently indicating the presence
of breakages.
FIG. 12A shows only a part of the through holes shown in FIG. 11,
but reinforcing electrode layers similar to the reinforcing
electrode layers 27-3-4', 27-2-4 are provided in the junction of
other through holes and the electrode layers shown in FIG. 11 and
similar to the reinforcing electrode layer 27-3-4 are provided in
other junctions the through holes.
In the stacked piezoelectric element of the present embodiment, the
piezoelectric layers have a thickness of 85 .mu.m while the through
holes have a diameter of 0.1 mm, the electrode layers 5, 6 have a
thickness of 2 to 3 .mu.m in average and the reinforcing electrode
layers have a thickness of 4 to 6 .mu.m. Although FIG. 12A
illustrates the electrode layers in an exaggerated manner, the
presence of the reinforcing electrode layers can be clearly
confirmed by observing the cross section of the stacked
piezoelectric element of the present embodiment.
The defective conduction between the electrode layers 25, 26 and
the through holes or between the through holes is prone to occur
particularly at the connecting portion between the end of the
through hole and the electrode layer, and such defective conduction
is caused not only by the polarization process but also by
defective filling of the electrode paste into the through hole
(particularly in the lower part of the through hole) or by
defective hot pressing, in a stage where the element is completed
for polarization, and the reinforcing electrode layers 27-2-4,
27-3-4 as shown in FIG. 12A are very effective for preventing such
defective conduction.
The above-described stacked piezoelectric element can be applied as
the vibration generating source constituting a vibration member in
a vibration driving device such as a vibration motor, in which a
traveling wave is generated by the synthesis of bending vibration
in two directions, but the application is not limited to such
object. It can also be utilized as the vibration generating source
for other purposes with an appropriate change in the configuration
of the electrode layers. It is for example sufficiently applicable
to a stacked piezoelectric transducer.
As explained in the foregoing, the fifth embodiment reduces the
defects caused in the polarization process, thereby improving the
production yield of the stacked piezoelectric element, and enable
the polarization process within a shorter time, without particular
change in the polarizing power source, voltage condition thereof,
or method or means for voltage elevation.
Also the producing method of the present invention for the stacked
piezoelectric element allows to easily form a reinforcing electrode
layer.
* * * * *